Alkaline peroxide treatment of rejects in an intergrated neutral-alkaline paper mill

ABSTRACT

A processes producing mechanical pulps utilizing refiners for improving the quality of screen rejects, and most particularly to a mechanical pulp mill integrated with a neutral-alkaline papermaking process producing mechanical printing paper grades is disclosed. The rejects are treated with hydrogen peroxide, an alkali and an organic stabilizing agent just prior to or during refining to provide improved optical and physical properties of the refined rejects and more efficiently utilize electrical energy to reach target fiber quality after bleaching for cost-effectively producing a wide array of coated and uncoated mechanical printing papers.

FIELD OF THE INVENTION

The present invention is directed to processes producing mechanical pulps utilizing refiners for improving the quality of screen rejects, and most particularly to a mechanical pulp mill integrated with a neutral-alkaline papermaking process producing mechanical printing paper grades. The rejects are treated with hydrogen peroxide, an alkali and an organic stabilizing agent to provide improved optical and physical properties of the refined rejects.

BACKGROUND OF THE INVENTION

Mechanical pulping is a process of mechanically triturating wood into its fibers for the purpose of making pulp. A key advantage of mechanical pulping is the high yield compared to chemical pulping which removes most or all of the lignin binding the cellulose fibers. Lignin is not removed from mechanically pulped wood, meaning scarce resources are more efficiently utilized. Pulps made using conventional mechanical pulping methods are mainly used for newsprint and other printing papers destined for direct mail advertising, directories and some magazines and books, and are typically unsuitable for higher quality or more durable paper end products. This is due, in part, to the fact that mechanical pulps are generally more difficult to bleach, can revert in terms of brightness due to light and humidity exposure and typically have lower strength than chemical pulps.

There are many variants of mechanical pulping including stone groundwood (SGW), pressurized groundwood (PGW), refiner mechanical pulp (RMP), thermomechanical pulp (TMP), and chemi-thermomechanical pulp (CTMP). The latter three can further be grouped generally under refiner pulping processes. During refiner pulping, wood chips are ground between rotating metal disks. The process usually is carried out in two stages. The first stage is mainly used to separate the fibers, while the second stage is used to modify the fiber surface for improved fiber bonding in the paper making process.

The foregoing list is by no means exhaustive. There are innumerable combinations and variants of the pulping processes. Of the mechanical pulping processes, the one which is considered by many in the field to be the most favorable, taking into consideration market conditions and environmental regulations, is the TMP process.

The TMP process typically consists of two refining stages. The first stage is pressurized, which allows the capture of thermal energy released as steam when mechanical energy is applied to the wood chips between the rotating refiner discs. Control of the steam pressure allows the primary refiner to operate at elevated temperatures and provides steam to heat and moisturize the wood chips before refining. The second stage refiner may be atmospheric, but it is more common to pressurize that stage also in order to capture more energy and better control the process. Pulps made by the TMP and CTMP processes have higher strength, which makes them the more favored mechanical pulping processes. However, there is still potential for improvement. The TMP process consumes a high quantity of electrical energy and the pulp produced by the TMP process tends to be darker than most other pulps due to thermal darkening of fiber during refining. Furthermore, the presence of large quantities of lignin in the refined fiber makes it highly susceptible to alkaline darkening.

In all mechanical pulping process, screen rejects are further processed to develop their favorable properties (e.g. bonding ability) and make them more attractive as fiber on the paper machine. Treatment of mechanical pulp rejects with sodium sulfite (reject sulphonation) prior to refining has demonstrated that strength improvements can be made. However, brightness losses during refining and significant increases in effluent BOD and COD as well as refining energy made it unattractive as a commercial practice.

The use of alkaline peroxide treatment of mechanical pulp rejects has been shown to produce improved physical properties and higher brightness equal to or superior to those of sulphonation. However, efficient alkaline peroxide treatments have been described as requiring a treatment time of from five to forty minutes or more to provide maximum improvements. See for example “HIGH ALKALINE PEROXIDE TREATMENT OF WHITE SPRUCE/LODGEPOLE PINE RMP REJECTS’, S. G. Book, pp 1-17, CPPA Pacific Coast Brach Mini-Conference April 1990 and “ALKALINE PEROXIDE TREATMENT OF SOUTHERN PINE TMP REJECTS”, M. J. Sferrazza et al., pp 617-629, 1988 Pulping Conference. In the described processes, 4.4-8.9% by weight NaOH based on fiber was being utilized to treat rejects in order to achieve the physical strength improvements noted and caustic/peroxide (C/P) ratios of 2-3:1 were utilized.

The use of alkaline peroxide treatment of rejects without a cooking or retention time in an RMP process was disclosed and found to be require caustic addition of 3.0% or higher by weight in “OPTIMIZATION OF IN-REFINER BRIGHTENING WITH H₂O₂ FOR PRODUCTION OF HI-BRITE MECHANICAL PRINTING PAPER”, V. Simard et al., pp 1-10, 1995 CPPA Spring Conference of Pacific Coast and Western Branches. The system described in Simard et al. employed a whitewater recycle stream of pH 5.0-5.5 to the TMP mill since the paper machine integrated to it was operating within that range.

More recently, improving TMP rejects and reducing refining specific energy through addition of alkaline peroxide solution prior to rejects refining was been discussed in conjunction with producing value-added grades of mechanical printing papers in “IMPROVING TMP REJECTS REFINING THROUGH ALKALINE PEROXIDE PRETREATMENT: AN OPPORTUNITY FOR SCA PAPER?”, Y. Bian et al, 2006 PAPTAC Pacific Coast Branch Spring Mini-Conference. However as in earlier published studies involving treatment of softwood fiber, total alkali application in excess of 5.2% by weight and reaction time of 30 minutes on high-consistency fiber at elevated temperature prior to refining were stated as required to achieve desired fiber quality (e.g. tensile strength, long fiber coarseness reduction) and operating cost improvements (e.g. refiner specific energy reduction, reduced chemical fiber requirement during papermaking).

US Patent Publication No. 2009/0032207 discloses a mechanical or chem.-mechanical process for making pulp in which, after fibrillation, the pulp is bleached in alkaline conditions. Thereafter, the rejects are screened and bleached separately from the accepts and the bleached rejects mixed with the accepts.

US Patent Publication 2008/0035286 discloses an alkaline peroxide mechanical pulping process which includes a step of treating fiberized lignocellulosic material with alkali peroxide chemical for a time and under conditions sufficient to obtain a pulp of desired consistency.

Bleaching is a term associated with a pulping process whereby certain chemicals are well mixed with fiber and then retained on the fiber for a given amount of time to increase the pulp's brightness. Bleaching is practiced on chemical and mechanical fiber pulps. In mechanical pulping, the increase in brightness is achieved by altering the chemical structure of the conjugated double bonds in lignin. The conjugated double-bonded species are called chromophores. “Brightening” is the term often used when referring to bleaching of mechanical pulps to distinguish it from the bleaching process of chemical pulps, which differs by the removal all lignin. As used hereinafter “bleaching” will be intended to cover the process of “brightening” as well.

In mechanical pulps, brightening is often carried out in a single step in the pulping process. The bleaching process is conventionally carried out in a bleaching train in one or a plurality of vessels (bleach towers or stages) in a distinct section of the mill, as opposed to the pulping section of the mill. Brightening can be carried out using oxidizing agents such as hydrogen peroxide and/or reducing agents such as sodium dithionite or sodium hydrosulfite.

Typically, hydrogen peroxide, an oxidizing agent, is used with sodium hydroxide. Sodium hydroxide is a strong alkali and provides the requisite high pH necessary to produce the active perhydroxyl ion, HOO⁻, thought to produce the bleaching effect in pulps. The cost of sodium hydroxide has been increasing due to changes in availability and energy costs. Concern over the environment has also meant a decrease in the available sodium hydroxide supply. Therefore, different alkali sources and different methods have been tried to find suitable alternatives for bleaching liquors and bleaching processes with limited commercial success.

To maximize the stability of ions of hydrogen peroxide and perhydroxyl during bleaching of mechanical pulp, sodium silicate and one of various organic and inorganic stabilizing agents are typically applied to fiber during and prior to addition of hydrogen peroxide and alkali on the fiber. These materials are beneficial largely due to their ability to control metal ions such as manganese, iron and copper that are contained in wood chips entering the pulping process. If untreated, these metal ions destroy hydrogen peroxide and perhydroxyl ion before it is able to brighten chromophores, making the process must less cost efficient and lowering brightening performance.

Although most previously published work related to in-refiner addition of alkaline peroxide onto screen rejects discussed addition of sodium silicate, the present invention specifically excludes the use of sodium silicate for stabilization of peroxide and perhydroxyl ion. The presence of silicates can result in the formation of scales (eg. calcium silicate, sodium carbonate) such as on refiner plates which can limit the ability of the refiner to refine rejects. The exclusion of the use of sodium silicate in the present invention avoids this potential problem

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a bar graph of ISO Brightness of Refined Screen Rejects based upon Example 1.

FIG. 2 is a graph of ISO Brightness of Refined Screen Rejects versus % Peroxide based upon Example 1.

FIG. 3 is graph of Bulk of Refined Screen Rejects versus Caustic based upon Example 1.

FIG. 4 is a graph of ISO Brightness versus Applied Peroxide in bleaching TMP Screened Stock based upon Example 1.

FIG. 5 is a graph of the % bulk decrease and % tensile strength increase compared to baseline from APTR at increasing C/P of Rejects Screen Accepts based upon Example 2.

FIG. 6 is a graph of the % Long Fiber due to APTR at increasing C/P of Rejects Screen Accepts and TMP Screened Stock based upon Example 2.

FIG. 7 is a graph of the % bulk decrease and % strength increase compared to baseline from APTR at increasing C/P of TMP Screened Stock based upon Example 2.

FIG. 8 is a graph of brightness measured on stock at various points through the TMP mill before, during and after a 7-day APTR trial at a 1.0 C/P based upon Example 2.

FIG. 9 is a graph of total specific energy in rejects refining versus final rejects freeness with compared to without APTR after a 7-day APTR trial at a 1.0 C/P based upon Example 2.

FIG. 10 is a graph of wet-end breaks by day for all downstream paper machines before, during and after a 7-day APTR trial at a 1.0 C/P based upon Example 2.

FIG. 11 is a graph of key pulp properties from pulp sampled at the Rejects Refiner Discharge after applying a moderate hydrogen peroxide dosage based upon Example 3.

FIG. 12 is a graph of key pulp properties from pulp sampled at the Rejects Refiner Discharge after applying a higher hydrogen peroxide dosage based upon Example 3.

DETAILED DESCRIPTION OF THE PROCESS

The present invention is directed to an improved process for the treatment of mechanical pulping rejects fiber wherein an alkaline hydrogen peroxide treatment is employed that does not require retention time on fiber prior to high consistency refining and which is effective at low chemical treatment levels. The use of zero retention time when adding alkaline treatment chemicals to thickened rejects prior to refining was found to be effective at caustic addition rates of less than or equal to 3.5% by weight in paper mills which operate at least one integrated neutral-alkaline papermaking machine and recycle water from this back to the mechanical pulp mill. As used herein, percentages are by weight unless specifically specified otherwise. Elimination of the requirement for retention of alkaline peroxide on rejects fiber prior to refining allows the present invention to be implimented with minimal capital cost. Recycle of neutral-alkaline paper machine white water to the mechanical pulp mill and the buffering associated with it from precipitated calcium carbonate (PCC) or ground calcium carbonate (GCC) filler pigment minimizes operating chemical costs by reducing the alkali demand of the mechanical fiber. This results in lower addition of total alkali on the rejects. The amount of hydrogen peroxide added when alkaline treating rejects is related to caustic additon so lower caustic requirements result in lower hydrogen peroxide requirements and makes the process more cost-effective.

With a ‘zero retention time’ scenario involving alkaline peroxide treatment of rejects (APTR) the caustic and hydrogen peroxide can be added directly to the rejects refiner or to the rejects pulp stream leaving the rejects thickener or to any available addition point between these two locations. To maximize brightness gain the caustic to peroxide ratio (C/P) is generally about 0.35-0.75. To maximize strength enhancement, C/P is generally about 1.25-1.5. A combination of increase in both brightnesss and strength can be achieved at intermediate C/P ratios based upon the particular needs of the specific installation.

As used herein, alkali is meant to include any source of alkalinity such as sodium hydroxide or caustic soda (NaOH), sodium carbonate (Na₂CO₃) and sodium bicarbonate (NaHCO₃). Na₂CO₃ and NaHCO₃ also provide buffer capacity to prevent wide swings in pH. When alkaline peroxide bleaching at high temperatures, better brightness is obtained with buffered systems. Buffering the system at lower pH (preferably between about 9 to about 10.5) prevents peroxide decomposition and darkening, but still provides adequate alkalinity to produce the desired species. The buffer releases alkalinity as necessary, and provides sufficient alkalinity for a slow and even production of the perhydroxyl ions. In whitewater from a neutral-alkaline paper machine that utilizes PCC or GCC as filler pigment, the calcium carbonate functions as a buffer and would be expected to improve brightening during APTR that occurs in a high temperature refiner. When added to fiber ahead of or in the rejects refiner, the components of the APTR liquor may be added separately or concurrently, concurrently meaning two or more components together such as a pre-mixed stream and separately meaning one at a time, as in individual streams.

When a premixed APTR treatment liquor is employed, a hydrogen peroxide stabilizer such as a suitable chelating agent may be included. Chelating agents can include, but are not limited to aminopolycarboxylic acids, (e.g.) ethylenediaminetetraacetic acid (EDTA), diethylene triamine pentaacetic acid (DTPA), nitrilotriacetic acid (NTA), phosphonic acids, (e.g.) ethylenediaminetetramethylene-phosphonic acid (EDTMP), diethylenetriaminepentamethylenephosphonic acid (DTPMP), nitrilotrimethylenephosphonic acid (NTMP), polycarboxylic acids, gluconates, citrates, polyacrylates, and polyaspartates or any combination thereof. A chelating agent may be added to the bleaching liquor in an amount up to 0.5% by weight based on fiber but is preferably added at addition rates of from about 0.1-0.25% based on fiber. As with all other components of the bleaching liquor, chelating agents may be added separately or concurrently with one or more bleach liquor components at one or more chemical addition points in the refining system. Chelating agents are thought to bind metals to prevent the decomposition of hydrogen peroxide which can cause darkening of the produced paper.

Treatment of a refiner rejects stream with alkaline peroxide without provisons for a holding period was found to provide increased strength and brightness at economically significantly lower caustic and hydrogen peroxide treatment levels for systems which utilize a recycled neutral-alkaline paper machine whitewater stream as dilution water in the mechanical pulp mill. By neutral-alkaline, it is meant a pH of about 7.0 to 7.5. Additionally, it was found that neutral-alkaline whitewater containing significant amounts of non-oxidized sulfur compounds leads to brightness gains during APTR that are lower than those where the neutral-alkaline whitewater does not contain these impurities.

It is now recognized that in a mechanical pulp mill with in-process dilution water derived from a paper machine or paper machines that operate at neutral-alkaline pH rather than at acidic pH, the requirement for caustic soda (and thus hydrogen peroxide) to achieve a given improvement in fiber qualilty can be lowered by up to 20%. This is due to the wood resin acids released in the high yield pulp during primary and secondary stage refining being somewhat neutralized by post-dilution with white water made up from the neutral-alkaline paper machine whitewater. Together, these discoveries create a process that allows neutral-alkaline mechanical printing paper mills to retrofit APTR into their mechanical pulping process with minimal associated capital cost. Further, it is now known that an APTR process operated in this manner is capable of eliminating the need for reductive brightening later in the process. This will result in a pulp of equivalent or superior optical properties and superior physical properties at the paper machine with a neutral to positive impact on furnish costs. Additionally, when pulp has been treated with the APTR process described herein, it can also be brightened in a conventional hydrogen peroxide bleach plant to produce a pulp having: a) superior optical and physical properties at a cost which is lower over an already achievable brightness range or b) equivalent optical and physical properties at an equivalent cost over a higher brightness range than currently achievable. Further, through APTR operated in the manner described herein, adverse effects on fiber properties such as yield and brightness losses are minimized between the mechanical pulp mill and the paper machine while simultaneously minimizing anionic charge loading to the paper machines and effluent BOD and COD loading. In addition, there is evidence that through APTR, rejects refiner specific energy versus freeness can be more closely controlled to realize operating savings.

EXAMPLES Example 1

Production trials were conducted at an integrated neutral-alkaline mechanical printing paper mill producing a range of paper grades having brightness specifications of 57.5-80% ISO and printing opacity specification of 91.5-96%. Fourteen trials were run which involved concurrently blending concentrated DTPA, NaOH and H₂O₂ into dilution water and adding this liquor briefly though a dilution water nozzle into the eye of a TMP rejects refiner. A chelant (DTPA) was additionally added ahead of rejects screen thickening at a constant addition rate of 1.5 kg/BDt.

Peroxide addition of 10, 20, 30 or 40 kg/BDt on a 100% basis was studied to determine the effects on optical and physical properties of fiber while the C/P ratio was varied at each addition rate of peroxide. DTPA addition rate on fiber remained constant. Table 1 summarizes the various combinations of DTPA. NaOH and H₂O₂ applied into the refiner.

TABLE 1 NaOH (C)/ NaOH, H₂O₂, DTPA, Trial # H₂O₂ (P) kg/BDT kg/BDT kg/BDT 1 0 0 1.0 2 0.5 5.0 10.0 1.0 3 0.75 7.5 10.0 1.0 4 0.5 10.0 20.0 1.0 5 1.0 10.0 10.0 1.0 6 0.5 15.0 30.0 1.0 7 0.75 15.0 20.0 1.0 8 1.5 15.0 10.0 1.0 9 1.0 20.0 20.0 1.0 10 0.75 22.5 30.0 1.0 11 1.0 30.0 30.0 1.0 12 1.5 30.0 20.0 1.0 13 0.5 20.0 40.0 1.0 14 0 0 1.0 15 0 0 0

Samples of refined rejects (including baseline samples that received no chemical addition) were blended with mainline accepts and the blend properties were determined. Several refined rejects-accepts blends were brightened with sodium hydrosulfite or hydrogen peroxide to determine bleach chemical savings. By applying 22.5 kg/BDt NaOH and 30 kg/BDt H₂O₂ to rejects, the tensile strength of accepts/rejects increased by 5-10%, the bulk of accepts/rejects decreased by 5-10%, the brightness of accepts/rejects increased by 4-6% ISO, the hydrosulfite used in brightening decreased by 100%, the caustic and peroxide used in peroxide brightening decreased by 10-15% and associated SO₂ for souring decreased as well since alkali and peroxide residuals after brightening were lower.

FIGS. 1-4 summarize the relationships observed after APTR on fiber from laboratory-prepared handsheets made from Refined Screen Rejects and brightened TMP Screened Stock samples.

Example 2

Trials were conducted at an integrated neutral-alkaline mechanical printing paper mill producing a range of paper grades having brightness specifications of 58-84% ISO and printing opacity specification of 85-97%. During extended operating periods of 12 hours and separately, 7 days, APTR was evaluated by applying a liquor containing dilution water, DTPA, caustic soda and hydrogen peroxide onto feedstock for two rejects refiners operating in parallel.

The 12-hour evaluations allowed: a) comparing results from two different addition points around the refiners, b) evaluating a low (0.6) and a medium (1.0) C/P ratio at a constant hydrogen peroxide addition of 30 kg/BDt to determine effects on fiber quality at various locations in the TMP mill that the rejects refiners were operating in and c) determining how rejects refining specific energy was impacted by APTR.

The 7 day and 24 hour per day evaluation were at a 1.0 C/P with addition of 30 kg/BDt peroxide on rejects and a) allowed paper machine whitewater coming back to the TMP mill to completely turn over, b) evaluated downstream effects of APTR in bleaching of TMP on the paper machines on various paper grades and c) provided data showing specific energy reduction in rejects refining.

Table 2 summarizes laboratory data of accepts samples collected during a single week of primary, secondary and rejects screens when two APTR operating periods of 12 hours each in duration could be compared to three operating periods surrounding them when APTR was not operating.

FIGS. 5-7 illustrates the key benefits of APTR in the TMP mill during the 12 hour trials while FIGS. 8-10 summarize key results of continuous automated testing in the TMP mill and at the three downstream paper machines over the extended 7 day trial.

During both the 12-hour and 7 days trials, sodium hydrosulfite for brightening TMP to low brightness was no longer applied downstream after operating APTR for 4 hours (when brightness and pH effects of APTR had stabilized compared to baseline). Further, after operating APTR for 4 hours, peroxide and caustic addition rates to the operating peroxide bleach plants were decreased by 50% of the equivalent of those being applied on TMP Screened Stock and further optimization to achieve 100% reduction and equivalent chemical costs was indicated through follow-up lab studies.

After 36 hours of operating APTR, brightness in the TMP mill began to drop and pH began to rise indicating that paper machine whitewater had completely turned over such that there was no longer a demand for alkali to neutralize TMP whitewater acidity. At this point, C/P was lowered from 1.0 to 0.9 where it stayed for the remaining 48 hours of the trial.

TABLE 2 Screen Accepts Samples Control Low C/P Control High C/P Control Screen Pri Sec Rej Pri Sec Rej Pri Sec Rej Pr Sec Rej Pri Sec Rej % Reject rate 52.0 63.8 na 51.7 64.9 na 49.3 63.4 na 51.4 64.8 na 56.5 72.8 na CSF na 72 89 68 71 96 68 67 95 68 71 95 59 54 96 0.004″ Pulrrac na 0.10 0.65 0.46 0.11 0.59 0.38 0.11 0.41 0.41 0.10 0.52 0.36 0.10 0.59 R14 na 2.2 11.2 1.1 2.0 13.0 1.7 2.0 11.0 1.9 2.3 15.4 1.5 1.3 10.2 14/28 na 31.0 38.8 30.0 31.8 38.3 30 31.1 39.5 30.4 32.8 38.0 28.8 28.2 40.0 28/48 na 12.3 11.5 12.9 12.9 11.2 12 12.7 11.2 11.1 12.6 10.4 11.6 13.2 11.1 48/100 na 10.6 10.2 13.9 10.7 10.2 13.9 10.6 10.4 13.7 10.5 10.0 14.9 11.5 10.5 100/200 na 4.1 4.4 6.7 4.7 5.0 7.2 5.0 5.7 6.9 5.1 5.2 7.6 5.8 5.6 P200 na 39.8 23.9 35.4 37.9 22.3 35.2 38.6 22.2 36.0 36.7 21.0 35.6 40.0 22.6 ISO pad brite na 53.4 54.6 54.9 54.3 63.6 52.7 52.3 53.5 54.2 54.4 60.3 53.0 53.7 54.3 % Opacity na 97.3 95.6 96.0 96.3 90.6 97.2 97.4 94.8 96.9 96.3 89.0 97.3 97.9 95.2 s coeff (557) na 65.5 55.8 64.7 64.1 53.0 65.9 65.8 53.6 66.0 64.6 45.9 67.0 69.4 54.4 k coeff (557) na 4.9 3.7 3.9 3.9 1.8 4.8 4.9 3.8 4.3 4.3 1.9 4.8 5.2 3.5 s coeff (457) na 75.1 57.5 61.3 63.2 54.3 65.1 66.6 57.4 62.7 59.2 46.2 65.0 59.1 52.9 k coeff (457) na 15.4 11.5 10.9 10.9 6.6 13.5 13.9 11.4 11.4 11.2 7.1 12.8 12.0 10.8 Bulk -cc/gm na 2.69 2.38 2.79 2.72 2.31 2.83 2.68 2.43 2.80 2.68 2.01 2.70 2.48 2.34 Tear Index na 7.8 8.8 7.8 7.7 9.1 7.4 7.4 8.5 7.4 7.9 7.2 7.5 7.7 8.2 Tensile BL in km 4.4 6.2 4.2 4.4 6.1 4.0 4.5 6.0 4.1 4.4 6.9 4.1 4.6 6.1

Example 3

Trials were conducted at an integrated neutral-alkaline mechanical printing paper mill producing a limited number of paper grades having brightness specifications of 58-60% ISO and printing opacity specification of 85-95%. During extended operating periods of 12-36 hours, APTR was evaluated by applying a liquor containing dilution water, DTPA, caustic soda and hydrogen peroxide into high-pressure dilution feed water added at the refining zone of one or two atmospheric refiners operating in series. These refiners process screened, cleaned TMP rejects that are subsequently bleached separately from additional IMP accepts/rejects with sodium hydrosulfite and utilized as a reinforcing fiber on two downstream paper machines.

FIGS. 11 and 12 illustrate the beneficial effects of APTR on brightness, tensile and other key pulp properties utilizing C/P ratios in the range of 0.35-0.40 in treatment liquor and DTPA applied into Unrefined Rejects prior to refining. 

1. A method for treating mechanical pulps comprising the steps of a) providing cellulosic materials derived from softwood or hardwood; b) introducing the cellulosic material to a mainline refining system for conversion to a mainline pulp; c) separating out rejects from the mainline pulp by means of screening or fractionation; d) mixing a neutral-alkaline whitewater stream, recycled from a paper process, essentially free of oxidizable sulfur species and containing a buffering agent, with said rejects to form a rejects fiber; e) adding a treatment liquor comprising hydrogen peroxide and an alkali to said rejects fiber, wherein the amount of alkali is less than about 3.5% by weight (100% basis) based on fiber and the ratio of alkali to hydrogen peroxide is from about 0.35/1.0 to about 1.5/1.0; and f) adding a chelant to control metal ions harmful to hydrogen peroxide stability to said rejects fiber and said neutral-alkaline whitewater stream.
 2. The method of claim 1, wherein said alkali is selected from the group consisting of sodium hydroxide, sodium carbonate or sodium bicarbonate.
 3. The method of claim 1 wherein said chelant is selected from the group consisting of aminopolycarboxylic acids, phosphonic acids, polycarboxylic acids, polyacrylates, polyaspartates, gluconates or citrates.
 4. The method of claim 1 wherein said alkali and hydrogen peroxide are added in combination with said rejects fiber ahead of refining.
 5. The method of claim 1 wherein said buffering agent is selected from the group consisting of precipitated calcium carbonate (PCC) or ground calcium carbonate.
 6. The method of claim 1 wherein said alkali and hydrogen peroxide are added in combination with said rejects fiber during refining. 